Classic 4x4 projective matrices, encoding the affine transformations of R^3.
Our convention is that they are intended for multiplication on
the right, that is, they are of the form

_____
[ | | 0 ]
[ | 3x3 | 0 ]
[ |_____| 0 ]
[ p q r 1 ]

Please note that by default, OpenGL stores the matrices (in memory) by columns, while we
store them by rows; but OpenGL also use the opposite convention (so the OpenGL projective matrices
are intended for multiplication on the left). So in effect, they are the same when stored in the memory,
say with poke :: Ptr Mat4 -> Mat4 -> IO ().